Monazite is the phosphate containing mainly ceric rare earths ((Ce, La, Nd…, Th) PO4) and is the main natural concentrator of thorium. It is widely spread (though usually in small amounts) in rocks and some types of ores. Owing to chemical and mechanical durability monazite is accumulated in placers.

The crystal structure of monazite can be presented as three-dimensional construction of oxygen nine apex polyhedron with rare-earth center atoms and oxygen tetrahedrons with the central atom of phosphorus. Nine-fold coordination allows a wide occurrence of relatively large ions of the light rare earths and thorium in mineral structure. The total content of thorium in a mineral can reach 28 wt%, and concentration of 5-7 wt% is usual. Though there are no experimental data about the form of radiogenic lead presented in the monazite structure, the numerous data, summarized for example in work [21], argued for its good stability in a monazite crystal matrix that allows monazite to be used for isotope dating.

In Ukraine monazite contains in developed fine-grained titanium-zirconium placers. By the explored easily enriched titanium-zirconium ores Ukraine comes to the forefront in Europe and in the CIS. The resources of zirconium in Ukraine make more than 10% of world ones. Now the largest Malyshevsky (Samotkansky) placer is developed and the working off of the Volchansky placer has been started.

Owing to the marked paramagnetism monazite at existing capacity of mines can be taken in passing by working out of placers in quantity of about 100 tons per year that corresponds approximately to 3.5 tons of thorium and 0.5 tons of the lead enriched with 208 isotope. Now monazite is considered as a harmful radioactive impurity and it is not produced.

The composition of monazite from the Malyshevsky placer as to the amounts of U, Th and Pb for dating purposes is well studied in work [21] by means of X-ray-fluorescent technique specially developed for individual grain analysis. In Table 3 the data about the contents of thorium, uranium and about isotope contents of lead for monazite of the Malyshevsky deposit is cited. The average composition of lead is confirmed by direct mass spectrometry determinations.

As is seen from Table 3, enrichment by 208Pb in the average for all monazite is insufficiently high. However, there is a probability of monazite separation by the flotation, magnetic or other characteristics with release of low uranium fraction of the mineral.

Extraction of total monazite concentrate by working out of the Malyshevsky placer scattering of an average almost won’t demand additional costs and its price as at first approximation can be accepted as the equal to zircon concentrate, i. e. ~ 1 US $/ kg. Cost of hydrometallurgical emanation of lead from monazite by analogy with similar processes can be estimated as (24^30 US $/ kg). The removal of differences with low U/Th ratio and the high content of 208Pb from monazite concentrate will require additional researches and will cause some rise in price of a product.

In Ukraine there are insufficiently studied shows of monazite in ancient radical breeds, their barks of aeration and in placers, i. e. enriched 208Pb. According to the available analytical data there is a possibility to detect monazite with highly enriched 208Pb.

For extraction of thorium and the lead enriched with 208 isotope Russia has a great opportunities by preparation the fine-grained titanium-zirconium placers for development and by the extraction from raw materials in complex deposits.

2. Conclusions

The paper is dedicated to the proposal of using lead enriched with the stable isotope 208Pb in FRs and ADSs instead of lead natural, natPb.

It seems that unique neutron features of 208Pb make it as one of the best among the molten metal coolants now assumed for FRs and ADSs: sodium, lead-bismuth, lead natural and others.

The main advantage of 208Pb is its low neutron absorption ability: for neutron energies En=0.1-20.0 MeV the microscopic cross sections of radiation neutron capture by 208Pb are by 1.5-2.0 times smaller as compared with natPb, and for energies, En<50 keV, the difference in the cross section values reaches 3-4 orders of magnitude. Averaged over neutron spectra of the LFR or ADS the one-group cross sections for a coolant from 208Pb are by 5-6 times smaller than those for the coolant consisted from natPb.

The second advantage of using 208Pb consists in achievement the core neutron spectra hardening on 5-6% due to low energy losses. Low neutron absorbing and moderating features of 208Pb permit to reach the gain in the multiplication factor Kef on 2-3% for critical or subcritical core fueled with U-Pu mix. In this case to have the multiplication factor Kef =1.01 for the LFR or Keff =0.97 for the ADS, both cooled with lead-208, the enrichment of power grade Pu in the U-Pu fuel can be reduced approximately on 0.7­0.8%.

The third important advantage of using 208Pb is coupled with increasing the small share of neutrons of low energies, 5-10 eV in spite of the neutron spectra hardening in whole. In this region of neutron energies the microscopic cross sections for such nuclides as 238U and 99Tc are maximum and very high, and the one-group cross sections for these nuclides averaged over neutron spectra of LFRs and ADSs cooled with lead-208 are equal to 0.6 and 0.8 barn respectively which are comparable with the one-group cross sections for typical breeders and transmutters.

The possibility of using 208Pb as coolant in commercial fast critical or subcritical reactors requires a special considering but relatively high content of this isotope in natural lead, 52.3%, and perspectives of using high performance photochemical technique of lead isotope separation permit to expect obtaining in future such a material in large quantities and under economically acceptable price. In the paper it is shown that principal possibility of acquisition of radiogenic lead containing high enriched lead -208, up to 93%, exists. Nowadays in Russian Federation and Ukraine thorium — containing loparit ores and monazite minerals are reprocessed for production of rare metal raw. Thorium and lead are not required now and they are deposited in sludge. Nevertheless, the scales of future thorium and radiogenic lead production for innovative nuclear reactors have some prospects in near-term future. The conclusion is made that to obtain the minimum amount of required in future radiogenic lead (65 t/year) for small sized FRs and ADSs the very large quantities of loparit ores or monazite minerals must be reprocessed and acquisition of radiogenic lead-208 can be economically acceptable as a co-product of rare metal raw.